Waveguide-to-coaxial line transducer



J11m- 1961 G. J. E. GOUBAU 2,970,284

WAVEGUIDE-TO-COAXIAL LINE TRANSDUCER Filed July 3, 1958 3 Sheets-Sheet 1ELEC , MAGNETIC y ENERGY ENTERING PORT ENERGY ENTERING PORT INVENTOR,GEORG J. E. GOUBAU.

. ir 2 d g? ATTORALEX Jan. 31, 1961 G. J. E. GOUBAU WAVEGUIDE-TO-COAXIALLINE TRANSDUCER 3 Sheets-Sheet 2 Filed July 5, 1958 ill l- INVENTORGEORG J. 5 sous/w.

9141M? 2% @an agmrg ATTORNEY.

Jan. 31, 1961 G. J. E. GOUBAU 2,970,284 WAVEGUIDE-TO- Filed July 3, 1958COAXIAL LINE TRANSDUCER 3 Sheets-Sheet 3 JIIIWIIIIIIIITW,

INVENTOR, GEO/5'6 J. 5. souanu "for this purpose.

A generalobjectis to improve transducers" fonelec'tri-WAVEGUIDE-TO-COAXIAL LINE TRANSDUCER -Georg J. E. Goubau, Eatontowm NJassignor to the UnitedStates-of America a's'representedby the Secretat-yof. the Army 7 Filed July 3,1958, set; No. 746,571

1 Claim; 01; ass-#34 (Granted under Title? 35', US: Code (1952 sec. 266)The invention described hereinrnay be manufactured and used by or forthe Government for governmental purposes, without the payment of anyroyalty thereon.

The-invention relates to guided electromagnetic wave transmissionsystems and particularly to transducers for use-in suchsystems to coupleelectrically transmission devices or lines having impedances ofdifferent types and values, such as a waveguide line and a'coaxial line.

netic waveenergy in the highfrequency range above about 50 megacyclcsper second hecauseof its extremely low attenuation over a wide band offrequencies in this range and extremely high power carrying capacity (H.H.

B'arlow The Relative Power-Carrying Capacity of High FrequencyWaveguides, Proc; IEE (London), Part E, vol. 90, pp. 21-27, January1952). As describedsin the aforementioned patent, the SFTL comprises anelongated con- "ductor such as a wire,. adapted to propagateelectromagnetic wave energy alongits outer. surface because of "specialconditioningofthat:surface, for example; by the provision of adielectric coating thereon, so as to reducethe phase velocity andthereby concentrate the field ofthe transmittedtw'ave" adjacent theconductor.

A SWTL is in effect a transmission device with coaxial terminations, andif his used" in" combination with a Waveguide transmission systemrequires a special adaptor -for properly electricallycoupling ittowav'egu'ide terminals in that system. 'Ihe-standard'adaptorswhich'have been provided for this purpose, which are usually designedfor connecting. SO-ohm cables, are not particularly suitable becausethey tend to reduce the usable frequency bandwidth oftheSWTL, Since theimpedance of the surface wave is in the order of 200 to 300 ohms, thelaunchers provided for exciting the surface wave inthe SWTL must takecare of an impedance transformation having a ratio of il or more. Thisnot only increases the costof the launchers but also reduces theircfiiciency because of inherent transformation losses. Even moreimportant is the fact that thecoa'xial termination of the standardadaptors limits the power-handling capacity of an associated surfaceWave transmission line particularly inthe centimeter-frequency rangewhere the physical dimensionsof the coaxial terminations are quitesmall. r

One object of the present invention is to provide an adaptor forcoupling a surface wave transmisison line;

Withfcoaxial terminations to the waveguide terminals in a waveguidetransmission system, which willnot have the above-mentioneddisadvantages of the prior art adaptors Patented Jan. 31, 1961 callycoupling transmission devices or lines having terminating impedances ofdifferent types and values in a high frequency signal wave transmissionsystem, from the standpoint of providing a more etficicnt transfer ofthe high frequency signal wave power between them while minimizingreflections at the junction point tending to increase power losses.

A related object is to provide at relatively low cost etficientimpedance matching at frequencies in the ultrahigh or super-highfrequency ranges, for example, at frequencies in the order of 500megacycles per second, between a coupled waveguide line and another linehaving a coaxial termination.

A more specific object is to provide efli'cient electrical couplingbetween a Waveguide. line and a surface wave or other. transmisison linehaving a coaxial termination ina high frequency signal wave transmisionsystem without appraciably reducing the power-handling capacity of thecoupled lines in the system.

The transducers in accordance with the invention for attaining theseobjects comprise two tandem-connected portions of differentconstructions. Both portions are three-port line junctionsbut one ofthem, to be referred to hereinafter as thefinput unction,,has threewaveguidecports and theother, to be referred to hereinafter as theoutput junction, has two waveguide ports and one coaxial port, The inputjunction operates to divide the electromagnetic signal waves entering aninput waveguide port' into two equal wave energy portions and topropagate those portions separately through-different ones of its twooutput ports consisting of two substantially straight sections ofhollow-pipe waveguide: of identical cross-sections and'having one sideWall in common, withthe electric fields of the propagated wave energyportions oppositelydirected with respect to the common wall.

The two input ports of the output junction are formed by two parallelsections of hollow-pipe waveguide with identical cross-sections andhaving one side wall'in' common. These two waveguide sections are alsoidentical in cross-section width, and are respectively connecteddirectlyto the two output waveguide ports of the input junction so astoreceive the propagated wave energy portions from the inputjunction.The output junction operates to combine these wave energyportions and toformcacoaxial'wave mode. The formation of this coaxial wave mode isachieved by' cutting away aportion of thecorninon wall of the adjacentwaveguide section constituting'the two input ports of the outputjunction and effectively extending the center'of this wall to the outputend-of the transducer, by, the'attachment thereto of a longitudinallyextending cylindrical conductor or wire to provide the inner conductorof the coaxial output port of which the extended outer. walls of the twowaveguide sections form the outerconductorj This coaxial output portprovides a high output. impedance: for the transducer and is adapted fordirect connection to the coaxial termination of the surface wave orother transmission line or device to'be electrically coupled through thetransducer to a waveguide line.

One feature of the invention is' the gradual cutting away of the commonWall of the'two adjacent waveguide sections of the output junction,forexample, in tapered fashion, to provide a gradual transformation ofthe electric field of the propagatedwave energy, in order to 'increasethe effective frequency bandwidth of the coaxial .guide junction in itsinput;

Figs. and 6 show perspective views, partially broken away to showstructural details, of different embodiments of the output junction of atransducer in accordance with the invention;

Fig. 7 shows a perspective view of a modification in accordance with theinvention of a portion of the output junction of Fig. 6;

Fig. 8 shows a perspective view of an embodiment of a completetransducer in accordance with the invention, in which the coaxial outputport of the output junction is derived in part from the rectangularwaveguide sections forming the input ports of that junction; and

Figs. 8A, 8B and 8C are cross-sectional views along the lines A-A, BBand C-C, respectively, of Fig. 8, showing the wall construction and thefield transformation at ditierent points along the transducer.

Fig. 1 shows a three-port input junction of a transducer in which theinput port 1 comprises one section of hollow-pipe waveguide ofrectangular cross-section and the two output ports 2 and 3 electricallycoupled thereto comprise two substantially straight sections ofhollow-pipe waveguide of identical rectangular cross-sections, having acommon side Wall 4. In Fig. 2, the input port 1A of the three-port inputjunction shown therein comprises a section of hollow-pipe waveguide ofcircular cross-section and the two output ports 2A and 3A coupledthereto comprise two substantially straight sections of hollowpipewaveguide of identical semi-circular cross-section, having a common sidewall 4A. Fig. 3 shows how the circular and semi-circular cross-sectionof hollow-pipe waveguide used for the input and output ports,respectively, in the input junction of Fig. 2 can be obtained by gradualdeformation of waveguides of rectangular crosssection. As indicated oneach of the figures, the input junction of Figs. 1, 2 or 3 operates todivide the electromagnetic wave energy entering the input port 1, 1A or1B into two equal energy portions which because of the.

particular orientation of the cross-sections of the connectingwaveguides are propagated separately through a different one of the twooutput ports 2 and 3, 2A and 3A, or 2B and 313, with the electricalfields E of the propagated wave energy portions oppositely directed withrespect to the common wall 4, 4A or 4B.

The particular embodiment of the transducer of the invention shown inFig. 4 employs the well known H-plane, T-type waveguide junction in itsinput. As shown, this junction comprises an input arm 5 comprising astraight section of hollow-pipe waveguide of rectangular crosssectionforming the stern of the T and two branch arms 6 and 7 consisting of twohollow-pipe waveguides of 1 identical rectangular cross-sectionsconnected at a com mon point to the output of the input arm 5, andforming the cross-arm of the T. This junction with the identicalcross-sections of the arms 5, 6, and 7 oriented as shown operates todivide the electromagnetic wave energy entering the junction through itsinput arm 5 into two equal energy portions which are separatelypropagated into difierent ones of the output arms 6 and 7 with theirelectric fields E oppositely directed. The extensions of the two brancharms 6 and 7 are bent inwardly as shown until near their output ends theadjacent walls combine so as to provide two straight sections 8 and 9 ofwaveguide having a common wall 10, constituting the two output ports ofthe input junction, similar to the two output 4 ports 2 and 3 in theembodiment of Fig. 1 and operating in similar manner.

Fig. 5 shows one embodiment of the output junction of a transducer inaccordance with the invention, in which the two input ports 11 and 12are formed by two parallel straight sections of hollow-pipe waveguide ofidentical cross-sections of semi-circular configuration, having onesidewall 13 in common. .In Fig. 5, a portion of the outer wall of thewaveguide sections 1 1 and 12 is broken away to show more clearly howthe coaxial output port of the output junction is constructed. Asindicated, part of the common wall 13 is removed and replaced by acylindrical conductor or wire l4 attached at one end to the center ofthe common wall and extending longitudinally to the output end of thetransducer. The conductor 14 forms the inner conductor of the outputcoaxial line section or port 15 for propagation of the coaxial wavemode, the outer conductor of which is provided by the extension of theouter walls of the two waveguide sections forming the input ports 11 and12 of the output junction, to the ouptut end of the transducer. Thedielectric disc 16 attached at a selected point along the cylindricalconductor 14 in the interior of the coaxial line output port 15 is usedto compensate for the impedance discontinuities which occur at thejunction of the conductor 14 and the common wall 13 by causing areflection of equal amount and degrees out-of-phase therewith, andthereby to effect complete transformation energywise of the waveguidemodes entering through the input ports 11 and 12 into the coaxial Wavemode leaving the output coaxial line section or port 15.

Although the dimension of the output coaxial line section or port 15usually would be sufiiciently large so that this section can transmitthe unwanted waveguide mode of the type TE the formation of which wouldlimit the size and thus the power-carrying capacity of the coaxialportion in standard waveguide-to-coaxial transducers, this mode wouldnot be excited in the output junction of Fig. 5 because of the oppositeorientation of the electric fields of the propagated wave energy withrespect to the common wall 13.

Fig. 8 shows one embodiment of a complete transducer in accordance withthe invention adapted for producing an increase in the effectivetransmission frequency band- With of the output junction. Referring tothat figure, the input junction of the transducer shown therein has aninput port 17 formed by a single section of hollow-pipe waveguide ofrectangular cross-section, open at one end and closed at the other by anaxially movable plunger 18, and two output ports 19 and 20 respectivelyformed by two substantially straight sections of hollow-pipe waveguideof identical rectangular cross-section and having one side wall 21 incommon. As shown, the inputs of the two Waveguide ports 19 and 20 areelectrically coupled to the input waveguide port 17 through a suitableopening in one sidewall of the latter at a point intermediate its ends,and the cross-sections of the three waveguide ports 17, 19 and 20 aresuitably oriented with respect to each other so that the electromagneticwave energy propagated over the input guide or port 17 and fed into thebranch waveguide sections or output ports 19 and 20 will be split intotwo equal energy portions which will be respectively separatelypropagated over a different one of these sections with the electricfields E of the propagated wave energy portions oppositely directed withrespect to the common wall 21 of these sections, as indicated inthefigure by the direction of the arrows so designated. The two input portsof the output junction of the transducer shown in Fig. 8 comprise theextended portions of the two waveguide sections 19 and 20 of identicalrectangular cross-sections. The coaxial output section or port of theoutput junction is formed in the following manner.

The full common wall 21 of the waveguide sections 19 and 20 extends onlyto a point 22 a distance in the order of a wavelength from the junctionof that wall with the :input guide 17. Beyond this ipoint the portion ofthe common wall 21 above and below a common center line are cut awaygradually in tapered fashion as shown. The taper preferably begins fromthe middle of the wall and extends to the output-end of the transducer,:or, :as shown, to the point 23 along the length of the guides 19 and20, a short distance from the output end of the transducer where allportions of the common wall 21 are eliminated and only the extensions ofthe outer walls of the guides 19 and 20 remain. A cylindrical rod orwire 24 having one end connected to the center of the common wall 21 atthe point 22 and extending longitudinally to the output end of thetransducer, forms the inner conductor, and the extended outer walls ofthe two waveguide sections 19 and 20 the outer conductor of an outputcoaxial line portion or port 25 of the transducer. The gradual (tapered)cutting away of portions of the common wall 21 in the manner describedresults in an increase in the bandwidth of the output junction. In thismanner, a more gradual transformation of the electric field of thepropagated waves can take place as illustrated by the field patternsshown in Figs. 8A, 8B and 8C.

The field transformation which takes place in the transducer when theelectromagnetic wave energy propagates from a cross-section A-A wherethe two wave energy portions are separated by the common wall 21,through the region where the wall 21 is cut away in tapered fashion sothat a varied amount of coupling occurs between the two energy portions,to the coaxial output of the transducer, is illustrated in thecross-sectional views of the transducer at dilferent point shown inFigs. 8A, 8B and 8C. The impedance matching produced by the transducerof Fig. 8 between a waveguide line connected to the open end of theinput guide 17 and any line with coaxial termination connected to itscoaxial output end 25 can be improved by proper positioning of theplunger 18 in the other end of the input guide 17.

Further improvement in the electrical coupling provided by a transducerin accordance with the invention can be obtained by reducing the phasevelocity of the waves in the waveguide sections to a velocity which isonly a little higher than the velocity in the coaxial line section ofthe output junction. One method of accomplishing this is illustrated inFig. 6 applied to an output junction employing waveguides 26, 27 ofidentical rectangular cross-section having a common wall 28, for the twoinput ports, which common wall is cut away gradually in tapered fashionalong its length beyond the point 29. A cylindrical conductor 30extending longitudinally from the center of wall 21 at point 29 to theend of the transducer forms the inner conductor of the output coaxialline section or port 31 of which the extended outer walls of thewaveguide sections 26 and 27 form the outer conductor. The requiredrelation between the phase velocities of the propagated waves in thewaveguide and coaxial line sections of the output junction is obtainedby the use of similar metal ridge members 32, 33 in the interiors of therectangular waveguide sections 26 and 27, which extend from thewaveguide sections into the coaxial line section and taper ofi graduallyin both directions. The ridge members 32, 33 may be located opposite tothe common wall 28 and attached to the outer walls of the guides 26 and27, respectively, as shown in Fig. 6, or may be attached to oppositesides of the common wall 28, as illustrated in the detail view of Fig. 6showing only the ridges 32, 33, the common wall 28 and the cylindricalconductor 30 forming the inner conductor of the output coaxial linesection or port. Instead of the metal ridge members, spaced dielectricslabs (not shown) in the interiors of the two waveguide sections 26, 27may be used for accomplishing the same purpose.

The parallel waveguide sections used for forming the input and outputports it of the input punction and the input ports of the outputjunction and for deriving the coaxial output port of the output junctionof the transducer, may

have configurations other than those illustrated in the embodiments ofthe transducer of the invention-shown in Figs. l'to 8. Also,although'only the use of the transducers ofthe invention fortransferring electromagnetic wave power in the direction from awaveguide line-to a line having a coaxial termination has been describedabove, it is to be understood that they are also applicable fortransferring electromagnetic wave power in the opposite direction, thatis from a line with coaxial termination to a waveguide line.

When a surface wave transmission line is one of the lines coupled by atransducer in accordance with the invention, the cylindrical conductorforming the inner conductor of the coaxial output port of the transducerwould be connected to the elongated conductor of the SWTL, and theoutput end of the outer walls of the extended waveguide sections formingthe outer conductor of that output port would be connected to the waveguide horn or other surface wave launching means such as described inthe above-mentioned patent, or the surface wave conductor may serve asconductor 30 of the coaxial section.

Various other modifications of the transducer arrangements illustratedand described which are within the spirit and scope of the inventionwill occur to persons skilled in the art.

What is claimed is:

A transducer for electrically coupling a waveguide line to another linehaving a coaxial termination comprising two tandem-connected linejunctions one having three waveguide ports and the other two waveguideports and one coaxial line port, the waveguide ports of said otherjunction being respectively connected in wave transmission relation witha diiferent one of two of the waveguide ports of said one junctionthrough two sections of hollow-pipe line of identical cross-sectionhaving a common sidewall between them, the connection between the threewaveguide ports of said one junction being such that the electromagneticenergy entering one port of that junction from said waveguide line issplit into two equal portions which are respectively propagated throughdifferent ones of said two sections to the other two waveguide portsthereof with their electric fields oppositely directed with respect tosaid common wall, said common wall being partly eliminated beyond apoint a given distance from the waveguide end of the transducer and acylindrical conductor is connected to the center of that wall at saidpoint and extending longitudinally to the coaxial line end of thetransducer to form the inner conductor of said coaxial line port, theextended outer walls of said line section forming the outer conductor ofsaid coaxial line port, the partial elimination of said common wallbeyond said point being provided by cutting away that wall in taperedfashion on opposite sides of a center line to provide a more gradualtransformation of the oppositely phased electric fields. to increase theefiective frequency bandwidth, the tapered cut beginning at the centerpoint to which said cylindrical conductor is attached and extendinglengthwise to the opposite outer edges of the common wall near thecoaxial line end of the transducer, impedance matching between thecoupled lines being provided by the use of a similar metal ridge sectionin the interior of each of said hollow-pipe waveguide sections attachedto corresponding wall thereof at a point opposite the common wall, whichridge sections taper olt gradually longitudinally in both directions, toreduce the phase velocity of the waves in the waveguide portions of thetransducer to a velocity which is only a little higher than that in thecoaxial line portion of the transducer.

(References on following page) 1 5 References Cited in the file of thispatent FOREIGN PATENTS H UNITED STATES'PATENTS 623,770 Great Britain May23, 1949 :Alford Nov. 10 1953 OTHER REFERENCES Smith May 14, 1957 5Ragan: Microwave Transmission Circuits, vol. 9, Worthington Dec, 24,1957 MIT Radiation Laboratory Series, copyright May 21, 1948 ElliottMar. 17, 1959 (pages 360-361 relied upon).

Robertson Feb. 9, 1960

